Method for eliminating traces of hydrocarbons from gas streams

ABSTRACT

The invention refers to a method for removing traces of hydrocarbons, particularly propane, from gas flows. The conversion of hydrocarbons into carbon oxides is achieved by loading suitable carrier materials, such as e.g. TiO 2  or Al 2 O 3 , with ruthenium as active component, possibly doping them with one or more further element(s), and subsequently calcining and/or reducing them at an increased temperature. By means of these catalysts, and at 20 to 150° C. and while adding molecular oxygen, hydrocarbons, particularly propane, in concentrations ranging from 0.1 to 2,000 ppm are oxidized.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage of PCT/DE01/02789 filed Jul. 19,2001 and based upon DE 100 37 165.5 filed Jul. 20, 2000 under theInternational Convention.

The invention refers to a catalytic process for removing traces ofhydrocarbons, particularly propane, from gas flows.

Catalysts for the total oxidation of hydrocarbons, which catalystsusually contain the platinum group metals platinum and palladium,possibly also rhodium, have been described already (J. Catal. 7 (1967)23, Catal. Today 54 (1999) 31, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980)293, J. Catal. 159 (1996) 361, J. prakt. Chem. 334 (1992) 465, U.S. Pat.No. 5,915,951). All these catalysts known so far have in common thatthey are active at temperatures of approx. 250° C. and above only. Thereare no known catalysts which convert hydrocarbons into carbon oxides atroom temperature already (Catal. Rev.-Sci. Eng. 29 (1987) 219). In aJapanese patent (JP 9113486), a method is described in whichhydrocarbons can be converted at 100–150°C. using high-voltage dischargecoupled with a Pt catalyst. However, this method requires a lot ofeffort.

EP-A-682975 refers i.e. to a mixed catalyst from Ag and Rh for removingof nitric oxids, co and hydrocarbons at temperatures of 150–650° C. U.S.Pat. No. 4,350,237 refers to the cataylic purifying of exhaust gases attemperatures of 200° C. DE-A-235137 describes mixed catalysts from i.e.Pt and Rh dor the purifying of exhaust gases with working temperaturesof 500° C. U.S. Pat. No. 3,931,050 describes mixed catalysts from Pt andRh for removing nitric oxides which catalysts are tested at temperaturesat 720° C. Weisweiler et al.(http://bwplus.fzk.de/berichte/Sber/PEF39005Sber.pdf) describes acatalytic decomposition of laughing gas.

The object of the invention is to provide a method in which propane andother hydrocarbons small amounts of which are contained in gas flows canbe converted into carbon oxides, preferably carbon dioxide, at lowtemperatures already.

According to the invention, the method for removing traces ofhydrocarbons from gas flows is characterized in that a gas flowcontaining traces of hydrocarbons in the range of 0.1 to 2,000 ppm ismade to pass over a catalyst having a specific BET surface of 1 to 1,500m²/g while adding molecular oxygen and at a temperature in the range of20 to 150° C., wherein the said catalyst, on the surface of anopen-pore, oxidic base body, contains compounds containing ruthenium,and wherein the ruthenium content is 0.1 to 20% by weight relative tothe total weight of the catalyst.

The catalyst consists of an open-pore, oxidic carrier material having aspecific BET surface of 1 to 1,500 m²/g, which material, on its surface,comprises compounds containing ruthenium, and wherein the rutheniumcontent is 0.1 to 20% by weight relative to the total weight of thecatalyst.

Advantageously, the carrier material is selected from the groupconsisting of titanium dioxide, titanium silicalite, aluminium oxide,alumosilicates, manganese oxides, magnesium oxide, acid zirconiumdioxide and mixtures thereof, and TiO₂ consisting of the modificationAnatas in the amount of 20–100% by weight is particularly preferred.

In another preferred embodiment, the carrier material is Al₂O₃.

Advantageously, the ruthenium content is in the range of 0.5 to 10% byweight, particularly preferred in the range of 0.5 to 5% by weight, andparticularly in the range of 0.5 to 3% by weight.

In addition to ruthenium, the catalyst may carry elements on thecatalyst surface which are selected from the group consisting ofplatinum, palladium, rhodium, gold, rhenium, bismuth, tellurium, lead,molybdenum, manganese, germanium, chromium, zinc, lanthanum, rare earthmetals and combinations thereof. It has been found that by means of suchadditives improved activities in the removal of small amounts ofhydrocarbons from gas flows could be achieved.

Particularly preferred additional elements, besides ruthenium, on thecatalyst surface are bismuth, lead, molybdenum, manganese, tellurium andchromium, alone or in combination with each other.

The catalyst is manufactured by applying ruthenium solutions onto thecatalyst surface, drying the catalyst precursor at temperatures in therange of 20 to 120° C., calcining the catalyst precursor in the presenceof oxygen at a temperature in the range of 200 to 600° C., and reducingthe catalyst in a hydrogen atmosphere at temperatures of 200 to 400° C.,or calcining and reducing under the said conditions.

In doing so, ruthenium(III) acetyl acetonate or aqueous ruthenium(III)chloride are preferably used as ruthenium solution.

Before drying, solutions of metal compounds may be applied, either atthe same time as the ruthenium solution or one after another, whichmetals are selected from the group consisting of platinum, palladium,gold, bismuth, tellurium, lead, molybdenum, manganese, rhodium, rhenium,germanium, chromium, zinc, lanthanum, rare earth metals and combinationsthereof.

In the method according to the invention, a preferred hydrocarboncontent is 10 to 2,000 ppm, and a preferred hydrocarbon is propane, forexample.

Advantageously, the propane may be contained in the gas flow in aconcentration of 0.1 to 1,000 ppm, particularly 10–1,000 ppm. Thetemperature at which the gas flow containing propane is brought incontact with the catalyst is particularly in the range of 50 to 150° C.

Preferably, the gas flow consists of air, or it contains air, whichsupplies molecular oxygen. The oxygen content should at least be so highas to guarantee a conversion of the hydrocarbons. It is preferred thatthe hydrocarbons be converted into carbon dioxide.

It is further preferred that the gas flow does not contain any nitrogenoxides.

Thus, the conversion of hydrocarbons into carbon oxides according to theinvention is achieved by loading suitable carrier materials, such ase.g. TiO₂ or Al₂O₃, with ruthenium as active component, doping them withone or more element(s), and subsequently calcining and/or reducing themat an increased temperature. By means of this measure, catalysts areprovided which oxidize hydrocarbons, particularly propane, attemperatures of 20 to 150° C. already. For example, at a catalystcontaining 3% by weight of ruthenium on titanium dioxide, propane (0.1%by weight in air) is converted into carbon oxides to the degree of 12%at 50° C., to the degree of 30% at 100° C., and to the degree of 81% at150° C., while the respective degrees of conversion at catalysts whichare manufactured in the same way, but contain other platinum groupmetals, are much lower (Pt: 3, 8 and 21%; Pd: 1, 3 and 14%).

The catalysts show a high activity at low temperatures (50–150° C.)already.

The invention will hereinafter be explained more precisely by means ofexamples. The surface measurements were carried out according to the BETmethod (Z.Anal.Chem. 238, 187 (1968)).

EXAMPLES 1–26

The manufacture of the catalyst precursor was carried out in two steps,wherein single steps or initial compounds do not apply if the respectivecompounds are not part of the catalyst. First, the porous carriermaterial TiO₂ (Degussa Aerolyst 7710, 0.25–0.5 mm, BET 49 m²/g, porevolume 0.88 ml/g) was impregnated with a mixture of aqueous solutions ofthe initial compounds H₂[PtCl₆], H[AuCl₄] and Mn(NO₃)₂, and dried at110° C. In a second stage, the materials obtained were impregnated witha mixture of aqueous solutions of the initial compounds (NH₄)₂PdCl₄,RhCl₃ and RuCl₃, and dried again at 110° C. The catalyst precursorsmanufactured in this way were calcined for 2 hours in an airflow (33ml/min per 200 mg of catalyst) at 400° C., and subsequently reduced for2 hours in a hydrogen flow (33 ml/min per 200 mg of catalyst) at 250° C.The catalytic test was carried out using 200 mg of the catalyst and agas mixture of 0.1% by weight of propane and 20% by weight of O² inhelium at a volume flow of 6 ml/min. Table 1 shows the compositions ofthe catalysts and the degrees of propane conversion at different reactortemperatures.

TABLE 1a Catalyst composition for Examples 1–26, carrier material: TiO₂(Degussa Aerolyst) Exam- ple Active components/% by weight No. Ru Pt PdRh Au Mn 1 1.72 0 0 0 0 1.28 2 2.35 0.65 0 0 0 0 3 1.00 0 0 1.10 0.91 04 1.40 0 0 1.60 0 0 5 2.41 0 0.59 0 0 0 6 1.93 1.07 0 0 0 0 7 1.66 0 0 01.34 0 8 1.17 0.20 0 1.15 0.47 0 9 1.33 0.24 0 0.76 0 0.67 10 3.00 0 0 00 0 11 1.17 0.20 0 1.15 0.47 0 12 2.35 0.65 0 0 0 0 13 2.01 0 0.99 0 0 014 1.92 1.07 0 0 0 0 15 0.61 0.21 0 1.19 0.99 0 16 1.39 0.24 0 1.36 0 017 1.52 0 1.49 0 0 0 18 1.43 0 0 1.57 0 0 19 1.91 0 0 1.09 0 0 20 1.00 00 1.10 0.91 0 21 1.01 0.18 0 0.99 0.82 0 22 0.98 0.89 1.07 0.06 0 0 230.58 0 0.79 0.68 0.95 0 24 1.44 0 0 0.83 0 0.73 25 1.17 0 0.99 0 0 0.8426 0.75 0 0 0.43 1.44 0.38

TABLE 1b Activity of the catalysts according to Examples 1–26 in theoxidation of propane (200 mg of catalyst, volume flow 6 ml/min, 0.1%propane, 20% O₂ in He) Exam- Degree of propane ple conversion/% No. 50°C. 100° C. 150° C. 1 11 37 84 2 7 34 84 3 12 33 68 4 12 33 68 5 4 32 816 9 32 83 7 10 31 80 8 16 30 70 9 9 30 60 10 12 30 75 11 4 30 67 12 1529 78 13 1 29 62 14 2 29 70 15 12 29 67 16 13 28 59 17 9 28 58 18 2 2572 19 1 24 60 20 5 24 65 21 1 24 56 22 2 22 58 23 3 22 66 24 4 21 55 255 21 61 26 1 20 58

EXAMPLES 27–29

Catalysts containing 3% by weight of Ru on different carrier materialseach were manufactured by means of an impregnation process analogous toExample 1, and tested for the oxidation of propane. The carriermaterials TiO₂ and Al₂O₃ resulted in catalysts which were active at lowtemperatures already.

TABLE 2 Influence of the carrier material on the activity in theoxidation of propane (3% by weight Ru/carrier each; 200 mg of catalyst,6 ml/min, 0.1% propane, 20% O₂ in He) Exam- Degree of propane pleconversion/% No. Carrier 50° C. 100° C. 150° C. 27 TiO₂ (Degussa 12 3081 Aerolyst) 28 TiO₂ (Degussa P25) 5 27 59 29 Al₂O₃ (Kalichemie 9 19 28Aluperl)

EXAMPLES 30–49

Catalysts containing 3% by weight of Ru on TiO₂ (Degussa Aerolyst) weremanufactured by means of an impregnation process as described inExample 1. After drying, they were doped with a second metal in theamount of 0.3% by weight each by means of impregnation with aqueoussolutions of metallic salts, dried again, calcined and reduced. In theoxidation of propane, the catalysts doped with Pt, Pd, Rh, Au, Mn, Re,Bi, Te, Mo, Pb or rare earth metals showed an increased activity at lowreaction temperatures compared to the non-doped Ru catalysts.

TABLE 3 Catalyst composition and activity of doped Ru/TiO₂ catalysts inthe oxidation of propane (200 mg of catalyst, 6 ml/min, 0.1% propane,20% O₂ in He) Exam- Degree of propane ple conversion/% No. Activecomponents 100° C. 30 3% by weight Ru 30 31 3.3% by weight Ru 32 32 3%by weight Ru, 0.3% by weight Pt 35 33 3% by weight Ru, 0.3% by weight Pd37 34 3% by weight Ru, 0.3% by weight Rh 36 35 3% by weight Ru, 0.3% byweight Au 36 36 3% by weight Ru, 0.3% by weight Mn 41 37 3% by weightRu, 0.3% by weight Re 39 38 3% by weight Ru, 0.3% by weight La 33 39 3%by weight Ru, 0.3% by weight Ce 34 40 3% by weight Ru, 0.3% by weight Nd36 41 3% by weight Ru, 0.3% by weight Sm 33 42 3% by weight Ru, 0.3% byweight Gd 34 43 3% by weight Ru, 0.3% by weight Bi 54 44 3% by weightRu, 0.3% by weight Te 42 45 3% by weight Ru, 0.3% by weight Mo 40 46 3%by weight Ru, 0.3% by weight Pb 42 47 3% by weight Ru, 0.3% by weight Ge30 48 3% by weight Ru, 0.3% by weight Cr 43 49 3% by weight Ru, 0.3% byweight Zn 31

EXAMPLES 50–57

Catalysts containing 3% by weight of Ru on TiO₂ (Degussa Aerolyst) wereprovided with different amounts of Mn by means of an impregnationprocess, as described in Examples 30–49. The catalysts containing Mnwere considerably more active in the oxidation of propane than thecatalysts containing solely Ru, wherein a maximum activity was achievedat Mn contents of 3.0% by weight and more.

TABLE 4 Catalyst composition and activity in the oxidation of propane(200 mg of catalyst, 6 ml/min, 0.1% propane, 20% O₂ in He) Exam- Degreeof propane ple conversion/% No. Active components 50° C. 100° C. 150° C.50 3% by weight Ru 14 28 69 51 3% by weight Ru 13 26 61 52 3% by weightRu, 17 27 71 0.01% by weight Mn 53 3% by weight Ru, 17 33 78 0.1% byweight Mn 54 3% by weight Ru, 17 46 95 0.3% by weight Mn 55 3% by weightRu, 19 41 87 0.7% by weight Mn 56 3% by weight Ru, 17 43 94 1% by weightMn 57 3% by weight Ru, 18 45 86 1.5% by weight Mn

EXAMPLE 58

2.39 g of ruthenium(III) acetyl acetonate were dissolved in 650 ml oftoluene, and added to 60 g of Al₂O₃ (Degussa Aluminiumoxid C) whilestirring. After the mixture had been stirred for 1 hour at 20° C., itwas left at room temperature for several days until the solvent hadevaporated. 200 mg of the catalyst were calcined in air for 2 hours at400° C., and subsequently reduced in a hydrogen flow for 2 hours at 250°C. In the following test, 44% of the propane were converted into CO₂ at50° C., 52% at 100° C., and 80% at 150° C. A long-term test at 22° C.showed that the catalyst worked for 8 hours without any loss of activity(Table 5).

TABLE 5 Long-term test at 22° C. using 1% by weight of Ru/Al₂O₃according to Example 58, activity in the oxidation of propane (200 mg ofcatalyst, 6 ml/min, 0.1% propane, 20% O₂ in He) Time/h X/% 0.5 38 1.0 411.5 43 2.0 42 2.5 42 3.0 43 3.5 43 4.0 42 4.5 42 5.0 41 5.5 42 6.0 426.5 42 7.0 41 7.5 42

1. A method for removing traces of hydrocarbons from gas flows: passing a flow of a nitric oxide-free gas containing traces of hydrocarbons in the range of 0.1 to 2,000 ppm over a catalyst while adding molecular oxygen and at a temperature in the range of 20 to 150° C., wherein said catalyst comprises a carrier material having a specific BET surface area of 1 to 1500 m²/g and at least one catalytic component on the surface of said carrier material, said at least one catalytic component is selected from the group consisting of: a) a share from 0.1 to 20% by weight of ruthenium relative to the total weight of the catalyst; and b) ruthenium and elements selected from the group consisting of bismuth, lead, molybdenum, manganese, tellurium, chromium, and combination thereof.
 2. A method according to claim 1, wherein the carrier material of the catalyst is selected from the group consisting of titanium dioxide, titanium silicalite, aluminium oxide, alumosilicates, manganese oxides, magnesium oxide, acid zirconium dioxide and mixtures thereof.
 3. A method according to claim 2, wherein the carrier material is TiO₂ having the modification anatase in the amount of 20–100% by weight.
 4. A method according to claim 2, wherein the carrier material is Al₂O₃.
 5. A method according to claim 1, wherein the ruthenium content is in the range of 0.5 to 5% by weight, particularly 0.5 to 3% by weight.
 6. A method according to claim 1, wherein the content of traces of hydrocarbons in the gas flows is in the range of 10 to 2,000 ppm, preferably 10 to 1,000 ppm.
 7. A method according to claim 1, wherein oxygen in the form of air is supplied to the gas flow.
 8. A method according to claim 1, wherein the hydrocarbons contained in the gas flows comprises propane in a concentration of up to 1,000 ppm, and the temperature is maintained in the range of 50 to 150° C.
 9. A method for removing traces of hydrocarbons from gas flows comprising: passing a flow of a nitric oxide free gas containing traces of hydrocarbons in the range of 0.1 to 2,000 ppm over a catalyst while adding molecular oxygen and at a temperature in the range of 20 to 150° C.; wherein the catalyst includes a base body of TiO₂ and at least one catalytic component on the surface of said base body, said base body has the modification anatase and a specific BET surface of 1 to 1,500 m²/g and said at least one catalytic component is selected from the group consisting of a) ruthenium from 0.1 to 20% by weight relative to the total weight of the catalyst; and b) ruthenium and elements selected from the group consisting of bismuth, lead, molybdenum, manganese, tellurium, chromium, and combination thereof. 